1. Field of the Invention
The present invention relates to a solid-state imaging device.
2. Description of the Related Art
FIGS. 1 through 3 of the accompanying drawings show a conventional solid-state imaging device 1.
The solid-state imaging device 1 comprises a plurality MOS transistors as pixels (hereinafter referred to as "amplifying pixel transistors") for storing electric charges, photoelectrically converted from applied light, under gate electrodes, and modulating and amplifying drain currents with the stored electric charges.
As shown in FIG. 2, an N-type semiconductor layer 3 and a P-type well region 4 are formed on a P-type silicon semiconductor substrate 2. A ring-shaped gate electrode 6 made of thin-film polycrystalline silicon capable of transmitting light through a gate insulating film 5 of SiO.sub.2 or the like is disposed over the P-type well region 4. N-type source and drain regions 7, 8 are formed, by self alignment using the gate electrode 26 as a mask, on those portions of the P-type well region 4 which correspond to the central hole and outer circumferential area of the ring-shaped gate electrode 6. The ring-shaped gate electrode 6, the source and drain regions 7, 8, and associated components jointly make up an amplifying pixel transistor 9 which serves as one pixel. A passivated film 12 is disposed on inter-layer insulating films 14 that are formed on the amplifying pixel transistor 9.
As shown in FIG. 1, a plurality of such amplifying pixel transistors 9 are arranged in a matrix form. The source regions 7 of those amplifying pixel transistors 9 which are arranged as columns are connected to vertical common signal lines 10 provided by a first layer of aluminum, and the gate electrodes 6 of those amplifying pixel transistors 9 which are arranged as rows are connected to horizontal common vertical scanning lines 11 provided by a second layer of aluminum.
Heretofore, as shown in FIGS. 1 and 3, a wide contact pad 13 extends integrally from the ring-shaped gate electrodes 6 of horizontally adjacent two of the amplifying pixel transistors 9, thereby connecting these ring-shaped gate electrodes 6 to each other. The contact pad 13 is connected to each of the vertical scanning lines 11. The source region 7 of each amplifying pixel transistor 9 and the corresponding signal line 10 are connected to a source contact 19 (see FIG. 1). Each contact pad 13 and the corresponding vertical scanning line 11 are connected to a gate contact 20.
As shown in FIG. 8, light that has passed through the ring-shaped gate electrode 6 generates electrons and holes h, and the holes h are stored as a signal charge in an Si--SiO.sub.2 boundary below the ring-shaped gate electrode 6.
When a high voltage is applied through the vertical scanning line 11 to the ring-shaped gate electrode 6 to turn on the amplifying pixel transistor 9, a drain current Id flows on the surface thereof. Since the drain current Id is varied by the signal charge h stored below the ring-shaped gate electrode 6, such a change in the drain current Id is supplied as an output signal through the signal line 10.
FIG. 4 of the accompanying drawings shows the conventional floating well amplifying imager in block form. As with MOS-type imaging devices, the floating well amplifying imager shown in FIG. 4 has a vertical scanning circuit 15 and a horizontal scanning circuit 16. The vertical scanning lines 11 connected to the gates of the pixel transistors 9 are connected to the vertical scanning circuit 15. The drains of the pixel transistors 9 are connected through power supply lines 17 to a power supply terminal t.sub.1.
The sources of the pixel transistors 9 are connected through horizontal switching MOS transistors 18 to an output terminal t.sub.2, and the gates of the horizontal switching MOS transistors 18 are connected to the horizontal scanning circuit 16.
In synchronism with television scanning lines, the vertical scanning circuit 15 increases the potential of an Nth vertical scanning line 11, for example, to turn on all the pixel transistors 9 in a horizontal row, and the horizontal scanning circuit 16 selects, or turns on, one of the horizontal switching MOS transistors 18 to select one of the pixel transistors 9, which have been turned on, as an ith pixel in the horizontal direction. In this manner, the pixel transistors 9 are selected in synchronism with the television scanning system, and the drain currents Id of the pixel transistors 9 which have been varied by stored signal charges h are detected and supplied as an output video signal.
With the conventional solid-state imaging device shown in FIG. 1, however, because the wide contact pad 13 extends from the ring-shaped gate electrode 6 of each of the pixel transistors 9 for connection to the corresponding vertical scanning line 11, the gate electrodes 6 are partly irregular in shape. Stated otherwise, the length of the gate electrodes 6 cannot remain constant in all areas thereof.
Therefore, the signal charges h stored in the gate regions tend to concentrate in the vicinity of the contact pads 13, and are also stored in the contact pads 13, resulting in degraded pixel characteristics.
The ring-shaped gate electrodes 6 are fabricated of a thin polycrystalline silicon film in order to allow light to be transmitted therethrough. Consequently, when the ring-shaped gate electrodes 6 are brought into contact with the vertical scanning lines 11 of aluminum through the contact pads 13, the silicon and the aluminum tend to chemically react with each other thereby alloying part of the ring-shaped gate electrodes 6 through the contact pads 13. Inasmuch as the alloyed regions have a work function different from that of polycrystalline silicon, the threshold value is varied partially by the difference between the work functions, a phenomenon referred to as a "potential shift", causing characteristic deteriorations.
An impurity may be introduced by ion implantation into regions beneath the contact pads 13 to produce channel stop regions for causing the signal charges h to be stored only in the ring-shaped gate electrodes 6. However, the signal charges may be stored irregularly within the ring-shaped gate electrodes 6 because of a mask misalignment.